A linear ion trap time-of-flight (“LIT-TOF”) mass spectrometer is a known device for analysing ions.
Typically, in an LIT-TOF instrument ions are trapped in a Linear Ion Trap (“LIT”), cooled and then extracted by application of an extraction voltage. The extraction voltage accelerates ions towards a time-of-flight (“TOF”) analyser. A TOF analyser is capable of undertaking mass analysis of the ions initially trapped in the LIT.
The present inventors have observed that if the pressure of a buffer gas in the LIT of a LIT-TOF instrument is too high, the performance of the instrument may be compromised due to the scattering of ions from buffer gas atoms/molecules during their extraction from the LIT. Furthermore, if the pressure in the LIT is high then the pressure in the TOF analyser maybe compromised, and/or additional pumping of the TOF analyser may be necessary. High pressure in the LIT may also result in fragmentation of ions, degradation of the TOF resolving power, transmission and the peak shape. High pressure may also lead to electrical breakdown when an extraction voltage is applied to extract ions from the LIT, thus limiting the magnitude of the extraction voltage that may be applied. This in turn will lower the maximum resolving power achievable.
If on the other hand the pressure is reduced in the LIT, the time for ions to reach thermal equilibrium with the background gas becomes longer, and consequently the time between the extraction of subsequent ion bunches from the LIT must be extended accordingly. In other words, the scan rate becomes low and this will result in degradation of the performance of the mass spectrometer, specifically the dynamic range, mass accuracy, sensitivity and the capability to follow dynamic events, e.g. when the molecular composition of the analyte changes rapidly in time.
U.S. 2010/072362A1 describes a segmented linear ion trap for receiving sample ions supplied by an ion source. A trapping voltage is applied to the segmented device to trap ions initially into a group of two or more adjacent segments and subsequently to trap them in a region of the segmented device shorter than the group of segments. The trapping voltage may also be effective to provide a uniform trapping field along the length of the device. The ion trap comprises a plurality of electrodes. The method of ion trapping taught by U.S. 2010/072362A1 uses a segmented linear quadrupole ion trap.
FIG. 1 is a simplified drawing of a linear ion trap 101 implementing the principles of U.S. 2010/072362A1, and a DC voltage profile 100 applied to the electrodes of the linear ion trap 101.
In this example, the linear ion trap 101 shown in FIG. 1 has seven segments, all having the same radial dimensions. A common RF voltage and individual DC supply is connected to each segment. The DC voltage profile 100 is applied to the segments for trapping ions in the central segment. The ion trap is filled with a buffer gas of uniform pressure. A collection of ions, having a range of m/z values are introduced from the left, i.e. into segment 1 at t=0 (not shown). Subsequently ions flow into all segments; after 3 ms the ion bunch has spread out and extends across the central 5 electrodes as indicated by reference numeral 111. By 7 ms ions have lost some energy, and ions of the ion bunch are collecting in the central (target) segment as indicated by reference numeral 112. At 10 ms the majority of ions of the ion bunch have collected in the target segment as indicated by reference numeral 113. At a certain time later the all ions would have reached thermal equilibrium with the buffer gas filling the trap. This allows for trapping ions with high efficiency into a linear ion trap. The invention is particularly useful when it is necessary that the target segment is at a pressure lower than could be used to trap ions in a single segment trap with high efficiency. By using e.g. five segments to form an extended region one is able to lower the pressure in the target trap by a factor of 5 compared to a single segment trap. A larger number of segments could be employed to attain efficient ion trapping at lower pressures.
The inventors have found that a LIT-TOF instrument constructed using the trapping method as described in U.S. 2010/072362A1 is effective to prepare ions clouds with the necessary properties to achieve good TOF resolving power.
However, one problem observed by the inventors with the trapping method of U.S. 2010/072362A1 is that a relatively large time using a relatively large LIT-TOF is required to trap and cool the ions at a safe operating pressure.
To illustrate this, we note that, for the purpose of LIT-TOF instrumentation a pressure in the target extraction trap of ˜1×10−4 mbar has been found by the inventors to be adequately low—the inventors have found no substantial advantage to operate at lower pressures in the case of extracting into a TOF of relatively short flight path (though there could be advantage for other types of analyser). Implementing the trapping method of U.S. 2010/072362A1, the inventors have estimated that one must employ ˜20 segments to achieve efficient trapping at the operating pressure of 1×10−4 mbar, assuming segments having an inscribed radius r0=2.5 mm and a length that is 8 r0. Thus, for a typical r0 of 2.5 mm, the total length of the LIT-TOF would be 400 mm. This is a substantial length, making instrument design inconvenient and expensive. Moreover, the time for ions to cool has been found to be relatively long, such that only a relatively slow scan rate can be achieved. In general, using the method illustrated in FIG. 1, the lower the required target pressure, the longer the cooling time will be, following a linear relation. Note that if the ions are extracted before the cooling process has completed, the inventors would expect a loss of resolving power and transmission in the TOF analyser to result.
U.S. Pat. No. 6,545,268 teaches an alternative method of trapping ions in a linear trap by a method of so called ‘dynamic trapping’ followed by collisional cooling.
FIGS. 2(a)-2(c) are simplified drawings of a linear ion trap 202 implementing the principles of U.S. Pat. No. 6,545,268.
The method of U.S. Pat. No. 6,545,268 provides a simple method to prepare a pulse of ions suitable for TOF analysis (see e.g. col. 6 lines 51-53). With reference to FIGS. 2(a)-2(c), ions are pulsed into the linear ion trap 202 having a buffer gas pressure, from an external source 201. The external ion source may be for example a multipole filled with ions 210. The ions 210 are initially prevented from entering the linear ion trap 202, by a DC voltage applied to the aperture 203, which is at a higher DC voltage than is applied to multipole 201 as shown by the DC voltage profile indicated by reference numeral 205 in FIG. 2(a). At some time later the voltage applied to aperture 203 is made lower than the voltage applied to multipole 201, as shown by the DC voltage profile indicated by reference numeral 206 in FIG. 2(b). Some of the ions in multipole 201 pass into the linear ion trap 202. Ions will pass into 202 and be reflected from the aperture 204, due to the electrical potential resulting from the voltage applied to it, which remains higher than the voltage applied to 202. Before any ions have time to pass back through aperture 203, the voltage applied to 203 is raised to prevent ions escaping, as shown by the DC voltage profile indicated by reference numeral 207 in FIG. 2(c). Ions are thereby trapped in linear ion trap 202, and reflect back and forth between apertures 203 and 204. After further time the ions have collisions with the buffer gas contained within 202. The pressure of the buffer gas within ion trap 202 will determine the length of time necessary for the trapped ions to reach a thermal equilibrium with the buffer gas. The lower the pressure the longer times is needed for ions to lose their energy and reach a thermal equilibrium with the buffer gas.
U.S. Pat. No. 6,545,268 has similar problems to those described above with reference to U.S. 2010/072362A1, although these are more severe and with additional issues of mass discrimination. In particular, a relatively long period of time is required to cool the ions following trapping within the linear ion trap 202 (as for U.S. 2010/072362A1). Mass discrimination arises due to the fact that ions with lower m/z values have higher velocity than those with higher m/z, and hence enter and reflect back from the aperture 204 more quickly than those with higher m/z. The ratio of the highest and lowest m/z ion which may be trapped effectively in this manner is hence defined by the length of the linear ion trap 202 and the energy at which ions are admitted to the linear ion trap 202. In practical situations, this length and energy combination for the apparatus shown in FIGS. 2(a)-2(c) are likely to limit the ratio to perhaps ˜3: For example, if the lowest m/z ion trapped is m/z 50, then the highest m/z ion which may be trapped might be m/z 150. The range which may be trapped may be altered by altering the time for which the low voltage is applied to aperture 203, but each fill of the linear ion trap 202 would trap ions which exhibit a considerably lower mass range than the maximum which might be trapped by a linear ion trap (for example, a mass range of 10 or more times may be achievable).
In view of the above considerations, the inventors believe it may be desirable to devise an ion trap, preferably a linear ion trap, capable of trapping ions efficiently whilst cooling them quickly to be in thermal equilibrium with a buffer gas contained within the linear ion trap, whilst the gas pressure existing in the target segment is typically less than 5×10−4 mbar and more preferably less than 2×10−4 mbar.
The present invention has been devised in light of the above considerations.